Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 3, pp. 287-290, 2005
Transmutation in Reactor and Aqueous Corrosion Resistance of Technetium Metal
J. M. Bonnerot,*, a V. Broudic,b M. Phélip,a C. Jégou,b F. Varaine,a X. Deschanels,b M. F. Arnoux,b
J. L. Faugèrea
a
b
Nuclear Energy Division, CEA Cadarache Center, 13108 Saint-Paul les Durance Cedex, France
Nuclear Energy Division, CEA Rhône Valley Center, BP 17171, 30207 Bagnols sur Cèze, France
Received: July 31, 2005; In Final Form: September 30, 2005
In the context of the French nuclear waste management act of 1991, two options have been considered following
enhanced reprocessing of spent fuel. The reference option is the transmutation by neutronic bombardment, the
second option is to immobilize the separated radiotoxic elements in dedicated matrices ensuring long term stability. This paper summarizes the research and development activities carried out by CEA in France, concerning
technetium. Nearly 90% of the technetium-99 present in spent fuel is separated in the UP2-800 reprocessing plant
at La Hague, producing about 900 kg of Tc each year. The first part of the paper deals with the irradiation experiment ANTICORP 1 which aims at transmuting 99Tc under the form of metallic rods, in the core of the Phénix
reactor. The goal of ANTICORP 1 is to achieve a transmutation rate of 99Tc in 100Ru greater than 20%. The main
options chosen for the design and the safety study of the experiment are described. In addition, the planning of the
irradiation and the post-irradiation exams foreseen, are detailed. The second part of the paper describes the studies carried out to investigate technetium metal as a potential containment matrix. The corrosion rate of this metal
under water was measured over one year of alteration at 25 ˚C.
1. Introduction
gate technetium metal as a potential containment matrix.
In the context of the French nuclear waste management act
of 1991, two options have been considered following enhanced
chemical separation during spent fuel reprocessing. The reference option, transmutation by neutron bombardment in nuclear
facilities, would reduce the initial radiotoxic inventory. The
second option is to immobilize the separated long-lived radionuclides in dedicated matrices ensuring long term stability in
geological disposal.
Long-lived fission products tend to exhibit greater solubility
in the environment than the actinides, and could contribute more
significantly to the residual radiotoxicity of a disposal repository. Three fission products have been studied in greater detail:
99
Tc, 135Cs and 129I. Although the beta radio toxicity of 99Tc is
far below the alpha radio toxicity of minor actinides, its high
mobility and very long life (half-life of 2.11 105 y) increases significantly the very long-term risk during storage. Technetium is
produced in the reactor mainly as 99Tc by neutron capture in
molybdenum arising from 235U and 239Pu fission reactions. The
quantity of technetium-99 to be conditioned from the reference
spent fuel (UO2 with a burnup of 45 GWd·t iHM−1) is 872 kg/year
based on a total of 800 metric tons spent fuel reprocessed per
year, the production capacity of the UP2-800 plant at La Hague.1
It is technically feasible to separate significant quantities (>
90%) of the Tc in solution by the UP3/UP2-800 process, although
about 10% of the Tc inventory is found in the form of insoluble
dissolution fines.1
A large research and development program is undergoing in
France2 since 1992, to demonstrate the feasibility of transmutation of minor actinides (americium, curium and neptunium),
which represent the majority of the long term radio toxicity elements in the waste, and also the main long-lived fission products
(technetium, iodine and caesium). The first part of this paper
deals with the main results obtained with Technetium. The second part of the paper describes the studies carried out to investi-
2. Transmutation of Tc in Reactor
*Corresponding author. E-mail: bonnerot @cea.fr. FAX: +33 4
42257042.
2.1. Irradiation Experiment ANTICORP 1 In Phénix. The
strategy of transmutation studies includes solutions based on
homogeneous recycling1 (with specific fuels) as well as heterogeneous recycling (with targets), in appropriate reactors relying
on current technology: Pressurized Water Reactor (PWR) and
Sodium cooled Fast Reactor (SFR).
ANTICORP 1 takes part of the irradiation program launched
in Phénix,3 and concerns the transmutation of 99Tc in a SFR.
From a neutronic point of view, a more efficient transmutation
can be obtained in moderated subassemblies in fast reactor, than
in standard PWR or SFR assemblies. Actually, such a design
combined a high neutron flux irradiation and an enhancement
of neutron epithermal captures for 99Tc (99Tc + n → 100Ru + β−).
Taking this design as a reference, ANTICORP 1 is positioned
inside a special rig locally moderated by mean of a CaHx moderator, and placed in the radial blanket of the Phénix core. CaHx
was selected because this material presents a high thermal stability and good moderation properties.
The ANTICORP 1 experiment will consist mainly in:
• Demonstrating the technical feasibility of the transmutation
of 99Tc with at least a transmutation rate of 20% and at a different temperature than previous irradiation experiments.
• Studying the conditions of 99Tc removal by transmutation in
stable 100Ru.
• Experimentally checking the validity of neutronic calculations.
2.2. Previous experiments. 4–6 Two transmutation experiments on 99Tc were carried out in the High Flux Reactor (HFR)
at Petten7, 8 in the frame of the EFTTRA collaboration including
CEA and European partners (ITU, FzK, EdF, NRG and IE). The
Tc was under the form of metallic rods (4.8 mm diameter) and
put inside a pin with a Phénix cladding and irradiated under a
thermal neutron flux in the HFR core. The irradiation conditions
of the two EFFTRA experiments are summarized in Table 1:
No significant changes in the characteristics of the targets
(dimension and microstructure) were observed after irradiation
© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2005
288 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005
Bonnerot
TABLE 1: Irradiation Conditions of EFFTRA T1 and T2
Experiments7, 8
Transmutation rate (at %)
Total fluence (m−2)
Fast fluence (m−2)
Temperature (K)
EFTTRA T1
EFTTRA T2
6
2.0 1026
8.8 1025
1100
15 to 18
5.4 1026
2.5 1026
1100
in the two cases. However, the Ru radial profile determined on
a section of the rod after irradiation shows a strong effect of
resonance shielding of the epithermal neutrons, which leads to
a sharp decrease of the concentration of Ru created in the outer
150 µm of the rod, that leveled off in the inner region.
These two experiments have demonstrated that no technical
limitations are foreseen to the use of technetium in metallic
form as a target for the transmutation.
More recently, metallic technetium targets-discs (φ = 6 mm,
th = 0.3 mm) have been irradiated in the high flux9 reactor SM3 of the Research Institute of Atomic Reactors (Russia). The
goal was to prepare artificial stable ruthenium pure enough for
non-nuclear industrial application. The transmutation rate has
achieved 34, 51 and 71% thanks to a high level of epithermal
neutron flux during irradiation (1.1 1014 cm−2s−1) and low thickness targets. This experiment performed on smaller sample
than EFTTRA T1 and T2 has demonstrated too, the relevance
of the transmutation of 99Tc metallic targets.
2.3. Irradiation device and Target. The irradiation device
consists in 3 pins, 100 mm long, each containing 2 ingots of
99
Tc in metallic form (6 g/ingot), encapsulated inside a stainless steel pin (Figure 1). The technetium rods of ANTICORP
1 were fabricated at the Institute for Transuranium elements
(ITU) at Karlsruhe, starting with powder obtained by reducing
ammonium pertechnetate (NH4TcO4) in a furnace at 873 K.
The fabrication method of the ingots is described in detail in
Reference 10. The main characteristics of the technetium rods
are: Diameter: 5.2 mm, length: 25 mm and density: 11.5 g.cm−3.
The cladding used for the pins is a standard Phénix cladding,
made of 15x15Ti stainless steel, with an inner diameter of 5.65
mm.
A pin holder basket (Figure 2) is used to set up the 3 pins in
radial position (pins separated by a 120˚ angle) and in axial position (Tc rods centered on the maximum flux plan of the Phénix
Upper plug Spring
Clad
Tc Ingot
core).
The capsule is a standard KMC used for experimental irradiation and internally cooled by a low sodium flow. The pin
holder basket is introduced and centered inside the body of the
capsule, as it can be see in the Figure 3.
The capsule is then placed inside the central hole of a DMC2 special irradiation rig that authorizes a local moderation of
the fast neutron flux. The neutron spectrum is shifted toward
the epithermal neutron zones by using a moderator material,
CaHx, placed inside the special rig at the level of the Tc targets.
This design allows rising the epithermal resonance neutron
capture of 99Tc during the irradiation.
2.4. Neutronic studies. In order to quantify the performances of the irradiation, different calculational methods were
used. Calculations were performed with the ERANOS system
associated with adjusted library ERALIB1 (based on JEF 2
evaluation). This system allows carrying out the whole neutronic characterisations. The adopted method is a heterogeneous calculation for the cell calculation in a fine energy mesh
discretization and a spatial calculation in transport method,
which takes into account photon propagation.
The self-shielded cross sections and scattering matrices are
processed for every core material using both the subgroup
method within each fine group and the slowing down treatment
in many groups (1968 groups).
The DMC-2 with ANTICORP 1 pins irradiation are described
in 2D cylindrical geometry for calculating the collision probabilities. The flux and current are calculated in a P1 approximation. The whole core calculations are processed in 33 energy
groups in transport method (Figure 4).
The x-axis decrease of the neutron flux inside the position of
the irradiation rig in the radial blanket is taken into account in
the 2D (X Y) calculations. This neutron flux decrease leads to
slightly different results for the three pins, depending on their
position inside the capsule.
The maximal transmutation rate obtained in the Tc rods during irradiation in the front pin (the nearest to the center of the
core of Phénix) is represented in the Figure 5.
As the maximal length of the irradiation will be 720 EFPD
until the final shutdown of Phénix, the maximum transmutation rate of Tc in ANTICORP 1 will reach 24.85% in the front
pin. The main values computed are reported in the Table 2.
The uncertainties to take into account on the fast neutron
fluence and the integrated dose are ± 19%.
Lower plug
Maximum Flux Plan
2570
Figure 3. Irradiation capsule.
25 mm
25 mm
spectrum in ANTICORP 1 pin (normalized to 1)
100 mm
0.08
Figure 1. ANTICORP 1 pin drawings.
Flux
0.06
0.04
0.02
0
100
10
1
0.1
1E-3
1E-5
1E-8
Energies (MeV)
Figure 2. Pin holder basket.
Figure 4. Spectrum (33 groups) in ANTICORP 1 irradiation.
1E-10
J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 289
Transmutation in Reactor and Aqueous Corrosion Resistance
Tc transmutation rate (at %)
30
25
20
15
10
5
0
0
120
240
360
480
600
720
Irradiation lengh (EFPD)
Figure 5. Evolution of the 99Tc transmutation rate during irradiation
in the front pin.
TABLE 2: Irradiation Conditions Calculated for ANTICORP 1 Pins
Transmutation rate (at %)
Total fluence (m−2)
Fast fluence (m−2)
Integrated dose (dpa Fe)
Front pin
Rear pin
24.85
1.3 1027
4.3 1026
18.2
24.53
1.2 1027
3.8 1026
15.7
The pseudo half-life of the 99Tc under irradiation (time corresponding to the transmutation of 50% of the 99Tc in 100Ru) is
closed to 1790 EFPD.
The power delivered in the Tc rod under irradiation results
mainly from the interaction of:
• Gamma from the Phénix core,
• gamma from radiative captures (E γ = 6.7 MeV),
• beta decay of 100Tc on 100Ru (Eβ = 1.315 MeV).
The different contributions to the total power delivered in
the Tc rods, calculated for the front pin (nearest of the core
center), are gathered in the Table 3. This table shows that the
major contribution to the power comes from the beta decay.
The uncertainties on the calculated linear power are ± 25%.
TABLE 3: Contributions to the Total Linear Power
Linear Power (W.cm−1)
Gamma from the core
Gamma (radiative capture)
Beta decay
Total linear power
3.3
5.3
8.8
17.4
2.5. Thermal studies. Modeling with the finite elements
code CAST3M was performed for the thermal design of the
pins. This modeling allows calculating the operating temperatures of the Tc rods and the cladding during irradiation. For
safety reason, the irradiation designer has to check that sufficient margin will be kept all along the irradiation, to prevent
from Tc rod melting or eutectic formation with cladding. For
the ANTICORP 1 design, the lower temperature to be considered for the safety study corresponds to the eutectic between
Nickel (element of the cladding) and Technetium, which occurs
at 1768 K.
The temperatures calculated in normal operating conditions
reach respectively 759 and 675 K in the Tc rods and the cladding. By taking into account all the uncertainties related to the
characteristics of the pins (geometry of rods and cladding,
thermal conductivity of the Tc and the filling gas, …) and the
irradiation conditions (linear power, coolant temperature, …),
the calculation shows that the temperature will never exceed
respectively 991 and 726 K. This result underlines that no risk
of melting is expected for ANTICORP 1 pins, in these irradia-
tion conditions.
As the evolution of the thermal conductivity, caused by the
Ru formation under irradiation (up to 25% Ru for 720 EPFD)
will be very slight,11 the temperature of the rods will be quite
stable until the end of the irradiation.
Compared to the EFTTRA experiments, the temperature of
the ANTICORP 1 Tc rods will be obviously lower (759 K
against 1100 K). These new irradiation conditions will allow
checking the influence of temperature of the technetium rods
on the behavior of the target under neutron flux (swelling in
particular).
2.6. Post irradiation exams. After irradiation the ANTICORP 1 capsule will be transferred and dismantled in the Phénix hot cells. Non-destructive exams, as diameter profilometry,
neutron radiography and gamma spectrometry, will be carried
out on the 3 pins. Then the 3 pins will be transferred to the
LECA facility in the Cadarache CEA center, to perform destructive exams. Only one pin will be cut to remove the 2 Tc rods,
and carry out at least the following exams: dimensional control, metallography exam, X-ray diffraction examination, micro
hardness, radial distribution of Ru control (EPMA) and chemical analysis.
2.7. Schedule. The irradiation of the ANTICORP 1 capsule
in Phénix started on June 2003 and will last until mid-2008 in
order to achieve the goal of 720 EFPD of irradiation. Up to
now (May 2005) more than 7.5% of the 99Tc (calculated data)
have been already transmuted in 100Ru. The post irradiation
exams are planned from the second half of 2008. As done for
the EFTTRA T1 experiment and if the targets reveal a good
behavior, CEA plans to extend the irradiation of the remaining
pins, in the frame of a further experiment in another reactor.
3. Aqueous Corrosion Resistance of Technetium Metal
3.1. Context of the studies. Various Tc immobilization
routes have been examined, including metal matrices such as
Nb alloys.1 In addition to these studies, and in view of the
properties of technetium itself, the aqueous corrosion resistance of pure Tc metal has been assessed.
Its solubility in aqueous media is highly dependent on its
oxidation state. Technetium exists at several oxidation states
between 0 and +VII. Under reducing conditions it is found at
oxidation state +IV and exhibits relatively low solubility (10−9
mol·L −1 at equilibrium with TcO2,12 whereas under oxidizing
conditions the stability of the TcO4 − ion enhances its solubility
and mobility.
3.2. Technetium Metal Alteration Process. Although
reductive redox conditions prevail in a deep geological repository site of the type under consideration in France,13 the possibility of oxidizing dissolution of technetium must be taken into
account because of the beta radiation field and water radiolysis
at the interface. Water radiolysis generates both oxidizing species (OH˚, H2O2, etc.) and reducing species (H˚, H2, etc.), and
can locally modify the redox conditions leading to the dissolution of the TcO4 − ion. The redox buffering capacity of the surrounding environment will be crucial for the existence of
oxidizing conditions at the Tc/H2O interface. The mechanism
considered is similar to the one discussed for dissolving uranium in spent fuel.14
The technetium dissolution reaction depends on the interface potential and on the possible presence of precipitates or
complexants in the system. A leaching experiment was first
conducted to monitor the Tc release versus time in pure water.
3.3. Experimental. Materials: This preliminary experiment was carried out on a piece of Tc metal taken from one of
the technetium rods prepared for the ANTICORP 1 experiment
for transmuting 99Tc subjected to a moderated flux in the Phenix reactor as part of the EFTTRA (Experimental Feasibility
of Targets for TRAnsmutation) project. The rods were fabri-
290 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005
Bonnerot
TABLE 4: Technetium Concentration Versus Time at 25 ˚C
in Pure Aerated Water (363 ac = acidified solution)
Time
days
Ai
Bq.mL −1
u(Ai)
Bq.mL −1
[Tcaq]
mol.L −1
1
7
10
36
91
197
363
363 ac
0,49
1,45
1,9
9,36
36,1
96,8
225,7
227,8
0,25
0,44
0,57
0,94
1,8
4,8
6,7
6,8
7,9E-09
2,3E-08
3,1E-08
1,5E-07
5,8E-07
1,6E-06
3,7E-06
3,7E-06
Ai × Vi
AS (Tc) × arod
y = 0,496x - 15,618
2
R = 0,993
150
100
50
0
0
100
200
300
Leaching time/days
Figure 6. Mean alteration rate of Tc metal in pure water at 25 ˚C.
4. Conclusion
More than 7.5% of the 99Tc have been already transmuted into
Ru in the ANTICORP 1 experiment in Phénix. At the end of
the irradiation, planned in mid-2008, a rate of transmutation
closed to 25% will be reached. ANTICORP 1 will make it possible to bring further information in comparison with EFFTRA
T1 and T2 experiments.
The mean alteration rate (0.49 ± 0.06 mg·m−2·d−1) for technetium over one year of alteration at 25 ˚C is encouraging in terms
of long-term behavior. A comprehensive investigation to identify the precise alteration mechanisms (oxidizing dissolution
under radiolysis, effect of dissolved oxygen in aerated media,
etc.) will determine whether this matrix is qualified for possible
use in a geological repository. A fuller understanding of the
alteration mechanisms will require the development of an
electrochemical approach suitable for investigating metal corrosion.
100
References
Mass loss (ML): The technetium mass loss at each sampling
time ti is estimated according to eq 1 :
MLi =
200
Mass loss/mg.m -2
cated in an arc furnace at the ITU in Karlsruhe and cast into a
cooled ingot mold. Their density exceeds 99% of the theoretical density (dth = 11.48 g·cm−3).15 The leached specimen was a
rod segment (m = 2.28 g, L = 9.56 mm, diameter = 5.2 mm,
S = 1.98 cm2) untreated except for decontamination with ethanol.
Leaching and analysis procedure: The specimen was leached
for one year at 25 ˚C in pure aerated water (V ≈ 100 mL) in a
PTFE Savillex container. Solution samples were taken at various intervals (1, 7, 10, 30, 90, 180 and 360 days) and analyzed
by beta counting to determine the Tc quantities released
(Eurisys IN 20 alpha-beta counter) and by chemiluminescence
(Turner Design TD20-20 luminometer) to detect any hydrogen
peroxide in solution.
3.4. Results and Discussion. The beta activity in solution
(Bq·mL −1) and the corresponding aqueous technetium concentration versus time are given in Table 4. After one year leaching, no adsorption of Tc was detected after acidification of the
solution in the vessel. Under the test conditions the technetium
showed very little corrosion; assuming the rod surface was uniformly altered, the mass loss after 1 year of aqueous leaching
represented a metal thickness of 0.015 µm.
(1)
Ai : Measured activity in the leaching solution
at ti in Bq.mL−1,
99
As(Tc): Tc specific activity (6.246.105 Bq.mg−1),
Vi : Leaching solution volume at t i in mL
arod : Tc sample surface area in m2 (geometric).
The values calculated for each sampling time and their combined standard uncertainties16 are plotted against time on
Figure 6. The dissolution of Tc seems to require an induction
period during which the release is very low and after which a
continuous and virtually linear technetium release into solution
is observed under these experimental conditions.
Alteration rate: Estimating the rate by linear regression from
the mass loss between 36 and 363 days yields a dissolution rate
of 0.49 ± 0.06 mg m−2·d−1 at 25 ˚C, with an uncertainty of twice
the standard uncertainty on the slope.
In comparison with UO2 spent fuel matrix, this rate of dissolution is one order of magnitude lower in simular experimental
conditions.11 Moreover, from a safety point of view, the reducing
conditions expected in a repository will probably limit the
migration of technetium into the near-field. In addition to the Tc
release, a hydrogen peroxide concentration of 7.10−8 mol·L−1 was
detected in solution, suggesting that water radiolysis occurred
under β irradiation.
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